The invention relates to a device according to the preamble of claim 1.
In pistons of internal combustion engines, a piston pin is usually mounted pivotally in appropriate lateral piston-pin bores in the piston on the underside of the piston head.
A piston pin mounted in this way is subjected to the undiminished bending stresses and strains of the forces generated by combustion in the combustion chamber and transmitted by the connecting rod. Accordingly, close tolerances must be maintained during production of the piston-pin mount, both during manufacture of the piston pin and from the viewpoint of the piston-pin bores. During assembly or subsequent removal of the piston for maintenance or repair, the piston can be inserted into the cylinder or withdrawn therefrom only together with the connecting rod, thus necessitating detachment of the oil pan. Only then can the lower connecting-rod bearing shell be removed. Assembly of the piston unit therefore requires extensive dismantling of the engine, and in this connection removal of the cylinder head is always indispensable.
In a known oscillating-shank piston (AT 340205), the piston pin is provided with lateral flattened areas, which are used as contact faces in order to bolt it together with the underside of the piston head. Laterally projecting bearing ends of the piston pin are mounted in corresponding piston-pin bores of the shank part of the reciprocating piston. The piston head is thus subjected to bending stress and strain in the normal manner. Since the piston pin has larger diameter in the region of its middle part than in the region of its lateral bearing surface the piston-pin bores are enlarged in the manner of oblong holes, in order to permit assembly of the piston pin.
In another known piston design (U.S. Pat. No. 5,307,732), a connecting member fixed inside the piston skirt is constructed in the manner of a shortened piston pin, which is accommodated on both sides in bearing shells on the underside of the piston head and bolted there. Such shell-like contact faces between connecting member and piston necessitate a relatively high manufacturing expense. This disadvantage is also found in another known piston design (DE 3235220), in which a further piston part is fixed inside the piston skirt, in which the connecting-rod bearing is disposed. To assemble the piston pin on which the connecting-rod eye is mounted, it is necessary to subdivide the internal piston part once more. Bolting guided through the piston head is used to fix these piston parts.
According to WO A 94/28297, finally, there is known a piston/connecting-rod unit which is provided with a connecting member integral with the piston having a surface for a connecting-rod eye, the connecting member being bolted together with the piston on the side thereof facing away from the combustion chamber, and the bearing surface being much broader at the straight portion of the connecting rod than at its side facing the piston. The piston comprises two parts, such that a piston skirt can be fixed together with a piston head by bolting between the latter and the connecting member. The connecting member has substantially cylindrical form and is braced with its upper circumferential face against a corresponding hollow shape in the underside of the piston head. This can indeed be removed upward in order to replace the piston rings; this is not true, however, for the piston skirt, which is joined via the connecting member to the connecting rod, in that it engages with its opposite end in piston-pin bores of the piston skirt. As a result of this lateral mount of the connecting member, it is subjected not only to tensile but also to bending stress and strain by the forces exerted by the piston.
Even in the embodiment of a piston unit of the same class according to GB A 2163520, the connecting member is so long that upward removal of the connecting rod is not possible when the bolting between the piston head and the connecting member is loosened. Annularly circumferential planar contact faces are provided in the bolting regions between the underside of the piston head and the connecting member, so that the piston skirt and a circumferential seal can be fixed by clamping in the bolting region. Since the bolts are seated in the lateral ends of the connecting member, and thus are disposed well outside the connecting-rod eye, the connecting member is subjected to bending stress and strain.
In contrast, the object of the present invention is to simplify the assembly/disassembly of piston and connecting rod and to provide between the straight portion of the connecting rod and the piston a connecting member which is suitable for permitting optimal dissipation of bending forces to the connecting member as a result of the forces exerted by the piston.
This object is achieved according to the invention in a device of the type-cited in the introduction, in the manner described in the body of claim 1.
Thus there is provided a reciprocating-piston engine with dividing planes between cylinder-head cover and cylinder as well as cylinder and crankcase, in which the usual piston pin is replaced by a connecting member which is bolted together with the piston on its side facing away from the combustion chamber and is provided with a bearing point for a connecting-rod eye as the piston-side pivotal bearing of the connecting rod, the connecting member being provided at least in the region of the bolting with planar contact faces, which form together with mating faces on the inside of the piston a common dividing plane, and that the connecting member is fixed by means of one or more connecting bolts passed through bores in the piston head. According to the invention, the bearing point is much broader toward the straight portion of the connecting rod than on its side facing the piston, the piston has one-piece construction and the connecting member is fixed in such a way inside the piston skirt that piston and connecting member can be removed through the cylinder opening on the cylinder-head side once the connecting bolts have been loosened.
In a one-piece embodiment of the bearing surface on the piston side, the connecting member is provided with two lateral contact faces, each adjoining the dividing plane, between which there is disposed the bearing surface on the piston side. In a split form of the bearing surface on the piston side, the connecting member has a central contact face, which adjoins the dividing plane and is disposed between two lateral bearing surface on the piston side. In both cases, the bearing surface on the piston side is much narrower than the bearing point directed toward the straight portion of the connecting rod, meaning that there is formed a stepped connecting rod with a narrow connecting-rod eye on the piston side and a comparatively much broader connecting-rod eye on the side of the straight portion of the connecting rod. In this way there is achieved optimal dissipation of bending forces to the connecting member by virtue of the forces exerted by the piston. With centrally disposed dividing plane and lateral bearing surface there exists practically zero bending load, because the connecting member does not project beyond the hole in the connecting rod, and so the firing forces are transmitted directly through the connecting member into the straight portion of the connecting rod. In this way loading of any kind is kept away from the piston skirt, and thus deformation of the piston skirt does not have to be feared. The piston can therefore be constructed on the whole in a particularly light and thin-walled form, meaning that the piston skirt can have correspondingly smaller structure. On the whole, a reduction of up to 50% compared with conventional embodiments can be achieved in the oscillating piston mass relative to the piston/connecting rod unit.
Once the cylinder head is removed, the connecting bolts, which are passed through bores in the piston head, become freely accessible from there. Preferably the bolt heads of these connecting bolts are countersunk in the head, for example in a head depression of the piston head. After the bolts have been loosened, the piston can be lifted upward, after which the connecting member can be removed, thus making the end (connecting-rod eye) of the straight portion of the connecting rod remote from the crankshaft freely accessible. The bearing shell of the connecting rod can then be loosened from the crankshaft very easily and in turn via the cylinder opening on the cylinder-head side. In the operation of removal of the bolts, the lower bearing cover of the connecting-rod bearing shell must be held by means of a tool, before it is removed with this tool.
Obviously the reverse procedure is followed for assembly of the connecting rod or of the piston, without the need to dismantle the engine. In other words, the oil pan does not have to be detached and the crankshaft does not have to be removed.
Regardless of the options toward simplification of assembly explained in the foregoing, the use of the connecting member according to the invention permits a reduction of manufacturing expense not merely because there is no longer any need for the expensive machining of the two lateral pin eyes in the case of a piston pin of conventional design; instead, the manufacture of the piston is further simplified to the effect that it is sufficient to bore this out centrally on the back side of the piston head, in order to create a planar face for fixing the connecting member according to the invention. Appropriate holes for the connecting bolts must be bored through the piston head, while the associated threaded bores are disposed in the connecting member.
The connecting member according to the invention is provided at least in the region of the bolting with planar contact faces, which together with mating faces on the inside of the piston form a common dividing plane. It is then expedient, for absorption of transverse forces in the region of the dividing plane, to provide straight pins that bridge it or fitted bushes that surround the connecting bolts; another option is to design the connecting bolts as close-tolerance bolts, the close-fitting portion of which then bridges the dividing plane.
According to a particularly advantageous embodiment, the device has only a single connecting bolt, which preferably is disposed at the center of the centrally disposed contact face and which is provided in the region of the dividing plane with a close-fitting extension, which is used for centering the connecting member relative to the piston. Such an embodiment permits considerable production and assembly simplification. In addition, there can be provided in the contact face an alignment pin, which together with the connecting bolt prevents the connecting member from being turned while the connecting bolt is being screwed in.
It is further expedient in this connection for a lubricating-oil bore disposed in the straight portion of the connecting rod to end at the bearing point of the connecting member on the connecting rod side, where a lubricating groove or flattened area of the surface of the bearing surface running in axial direction provides for distribution of lubricating oil.
In addition to distribution of lubricating oil in axial direction, there can also be provided a lubricating-oil groove disposed along a circumferential arc to direct sprayed oil onto the piston head. The spraying angle can be varied by increasing or decreasing the height of the shoulder of the connecting-rod eye that is axially oriented in the region of the bearing surface.